The present invention relates to the production of a fine-grained vinyl chloride suspension polymer and its use as a viscosity reducing agent (viscosity depressant) in polyvinyl chloride plastisol processing.
Polyvinyl chloride plastisols (polyvinyl chloride pastes) are generally understood to mean dispersions of fine polyvinyl chloride powders in plasticizers which have a not too strong swelling effect. Suitable paste-forming polyvinyl chloride grades for this use are emulsion polymers and, in particular, also microsuspension polymers. These are polyvinyl chloride products obtained in latex form after polymerization and processed generally by spray-drying into agglomerated secondary particles (plastisol grade). During plastisol processing, these agglomerates are predominantly decomposed into primary particles. The degree of such decomposition and the size distribution of the primary particles determine the flow properties of the paste.
Polyvinyl chloride pastes are utilized mainly as spread-coating pastes, as dipping and casting pastes, and as spray pastes for a great variety of finished articles.
In general, pastes having low viscosities are desirable for manufacturing finished articles having a low plasticizer content. It is known to lower the paste viscosity for this purpose by the addition of non-paste-forming, fine-grained suspension polyvinyl chloride products (so-called extender PVC) which are broken down only at the gelling temperature.
According to German Pat. No. 1,645,668, such suspension polymers for reducing the viscosity of polyvinyl chloride plastisols can be produced with the aid of methylhydroxypropylcelluloses, having a viscosity of 50-500 mPas in a 2% by weight solution at 20.degree. C., as the sole suspension stabilizer and with the use of monomer soluble catalysts. As can be seen from Comparative Example A (according to German Pat. No. 1,645,668 with a methylhydroxypropylcellulose as the suspension stabilizer which, in a 2% by weight solution, exhibits a viscosity of 100 mPas at 20.degree. C.), polymers produced in this way show, however, a considerable proportion of coarse grains having particle sizes above 100 .mu.m. This leads to an undesired sedimentation of the coarse proportion and therefore causes difficulties during processing. Furthermore, the coarse proportion results in an undesirable, rough surface, especially when very thin coats are applied.
According to DOS No. 3,018,940, these disadvantages are overcome by the use of a quite specific vinyl chloride graft polymer as the extender polyvinyl chloride. This involves vinyl chloride graft polymers manufactured in the presence of oil-soluble initiators, a suspension stabilizer, an emulsifier, and an ethylene-vinyl acetate copolymer. As can be seen from Comparative Experiment C of DOS No. 3,018,940 and Comparative Experiment B of the present application (Table 1), the particle sizes required for use as extender polyvinyl chloride in plastisols are obtained by adding certain amounts of emulsifier and/or tenside during polymerization. On account of the resultant grain fineness, such products do not show a settling tendency and are suitable for the application of thin plastisol layers. However, there is an attendant grave disadvantage. As can be derived from the above-mentioned Comparative Example B in Table 1, the addition of the tenside leads to a drastic increase in grain porosity; as a result, it is hardly possible to obtain a reduction in paste viscosity using this product. To counteract this increased grain porosity, polymerization is carried out according to DOS No. 3,018,940 in the presence of an ethylene-vinyl acetate copolymer. In order to improve the paste viscosity level attained without emulsifier, relatively large quantities of ethylene-vinyl acetate copolymer become necessary, resulting in a pronounced reduction of the gelling capacity of the thus-prepared plastisols and in a markedly higher turbidity of the gelled plastisol layers. As can be seen from Comparative Experiments C and D according to DOS No. 3,018,940, a small amount of ethylene-vinyl acetate copolymer effects hardly any reduction in paste viscosity.